How to export object with its texture for 3d printing?
I originally followed this tutorial where it outlines smart unwraping as part of the UV creation process. I started from scratch, and instead of doing the smart unwrap with island spacing, I just did an unwrap (more like one of the posts I looked at) and things now work as expected. The only downside is that some of the geometry has warped into very small triangles on the PNG file, but this isn't (in this case) a big issue.

PLY's still don't embed the data, but that's a problem for another day :)